NITROGEN NET MINERALIZATION UNDER FIELD CONDITIONS IN AN IRRIGATED SOIL.

Samples of a soil of known N-mineralization rates, previously determined under controlled moisture and temperature in the laboratory, were periodically collected from two plots that had been differently N fertilized for five years, receiving one twice as much N fertilizer as the other. Within each plot, sampling area was restricted to four microplots which were plantless and covered with black polyethylene sheet in a furrow irrigated potato field. Sampling at 20 or 30 cm depth increments was carried out to 120 cm depth. Moist and refrigerated samples were analyzed for mineral N and moisture. The plastic cover kept approximately constant soil moisture. Soil mean daily temperature was measured by thermistors, connected to a data logger equipment, placed at 2, 10, 30 and 50 cm depth.

Net organic N mineralization in the whole soil profile sampled for a 74 day-summer period were 131 kg N/ha and 113 kg N/ha for the normal and low fertilized plots, respectively. Contribution to those figures of the corresponding soil surface layer (0–20 cm) were 71% and 82%. Calculated field net N mineralization rates for the same period and surface soil ranged between 0.33 and 0.58 mg N/kg soil x day as compared to 0.37 to 0.42 mg N/kg soil x day obtained in soil samples in laboratory incubation experiment.